The invention is related generally to multifrequency band apertures, and more particularly, to apertures shared by two or more antennas, one or more of which is a phased array.
Multifrequency radiation and reception applications frequently are associated with space, weight, and mutual interference limitations. For example, applications on aircraft, spacecraft, ships at sea and mobile land platforms all typically have severe size and weight restrictions. It is typically impractical to have multiple antennas with multiple apertures in these applications. A shared aperture, wherein multiple antennas share a common aperture area, is preferred.
One type of shared aperture is the dual dipole aperture. Dipole elements for both frequency bands are used with a common ground plane. To minimize mutual coupling, the dipoles are orthogonally polarized. Because of the physical requirements of the dipoles, one set must be located behind the other set and must therefore, "see through" the more forward set. Typically, the higher frequency set of dipoles is disposed behind the lower frequency set. This arrangement results in pattern degradation for the higher frequency set because energy scatters off and couples to the interfering set of feed lines to the lower frequency dipoles. Also, because the spacing of the lower frequency set of dipoles is greater than one-half wavelength of the higher frequency set, impedance mismatch exists for the higher frequency elements and radiation in unwanted directions occurs. This radiation is commonly referred to as grating lobes or Bragg reflections and additionally results in a loss of power in the desired beam.
Combination waveguide/dipole shared apertures also exist with the waveguide containing the higher frequency energy. The dipoles are place in front of the waveguides with a similar result as described above for the two dipole arrangement. The lower frequency dipoles interfere with the energy of the higher frequency waveguides and grating lobes result.
In one prior technique where a single set of broadband elements is used for all frequency bands, the broadband elements are spaced at half-wavelength intervals at the highest frequency band. A multiplexer is used for each radiating element to separate out the various frequency bands. Because the elements are half-wavelength spaced for the highest frequency band, there are many more elements per wavelength for the lower frequency bands. It would be wasteful to use a phase shifter per element at the lower frequencies because only one phase shifter per one-half wavelength is required. Thus the outputs of the multiplexers should be combined in groups before the phase shifters to result in one phase shifter per one-half wavelength. This leads to a complex feed network, higher weight and larger size and is impractical for many applications.
Hence, those skilled in the art have recognized the need for a shared antenna aperture in which two or more sets of energy radiating elements for radiating different frequency bands may coexist in the same aperture without interfering with one another, in which grating lobes are minimized and which are more easily constructed than prior art apertures. The present invention meets these needs.